Phase IIIb, longitudinal, multicenter, randomized, open-label clinical trial to evaluate the efficacy and safety of the fixed-dose combination of etoricoxib 90 mg and tramadol 50 mg compared with fixed-dose therapy of acetaminophen 325 mg and tramadol 37.5 mg for the management of acute low back pain.

Patients with back pain from the level of the lowest rib to the gluteal crease, with or without radiation to the legs, as a first-time episode or after another at least 6 months before and lasting no more than 6 weeks, that in the opinion of the doctor requires treatment with the combination of an anti-inflammatory and an analgesic. Study to evaluate the efficacy and safety of the fixed-dose combination of etoricoxib 90 mg and tramadol 50 mg compared with the fixed-dose therapy of acetaminophen 325 mg and tramadol 37.5 mg for the management of acute low back pain. Evaluate and compare the average change in pain reported through the VAS at 3, 5 and 7 days with respect to its baseline measurement by treatment group. An exploratory analysis of the data will be carried out to identify aberrant, outliers, or missing values. To present the numerical variables, the measures of central tendency and dispersion appropriate to the distribution of the data will be used and the categorical variables will be expressed as frequency and percentage.

Inclusion Criteria:

• Any sex.
• That the subject agree to participate in the study and give informed consent in writing.
• Age> 18 years and ≤ 60 years of age at the beginning of the study.
• Diagnosis of acute low back pain as a first-time episode or after another episode, maximum 6 months before and lasting no more than 6 weeks.
• Patient with low back pain reported as moderate to severe intensity (VAS> 4 cm).
• Women of childbearing age who have an acceptable contraceptive method (eg barrier, oral hormonal, injectable, subdermal).

Exclusion Criteria:

• Patients in whom the study drug is contraindicated for medical reasons.
• Patients with an allergy or hypersensitivity to the active principle, study drugs, related products or excipients.
• Positive pregnancy test, women who are pregnant, breastfeeding or planning a pregnancy while conducting the study.
• A significant history of gastrointestinal disorders (for example: Gastric ulcer, Crohn's disease, Ulcerative Colitis, etc.).
• Previous treatment with opioids and / or NSAIDs (Non-Steroidal Anti-Inflammatory Drugs) reported in the medical history in the last 72 hours at study entry.
• Patients with a history of alcohol or drug abuse in the last year according to DSM-V (Diagnostic and Statistical Manual of Mental Disorders).
• Patients who are receiving monoamine oxidase inhibitors (MAOIs) or who have received them within the past 2 weeks.
• Patients with a history of established ischemic heart disease, peripheral arterial disease and / or cerebrovascular disease (including patients who have recently undergone coronary artery bypass grafting or angioplasty).
• Patients with status epileptic seizure disorders and grand mal seizures.
• Patients with a history of severe acute or chronic liver failure.
• Patients with a history of moderate to severe renal failure.
• Patient with a history of chronic pain (eg, fibromyalgia, Paget's disease, bone pain induced by metastatic cancer).
• Low back pain due to major trauma (eg vertebral fracture, post-traumatic spondylolisthesis), or due to a visceral disorder (eg dysmenorrhea, endometriosis).
• At medical criteria, a disease that affects the prognosis and prevents outpatient management, for example, but not restricted to: intestinal obstruction, paralytic ileus, end-stage cancer, kidney, heart, respiratory or liver failure or mental illness or with surgical procedures or scheduled hospitals.
• History / presence of any disease or condition that, in the Investigator's opinion, could pose a risk to the patient or confuse the efficacy and safety of the study results. ----Patients with symptoms suggesting an active COVID-19 infection (i.e., fever, cough, dyspnea) and / or contact in the past 14 days with a suspected or positive COVID-19 patient.
• Oncological patients (except basal cell skin cancer) or with serious diseases that, in the opinion of the investigator, have a serious prognosis or a life expectancy of less than 1 year, as well as mental illnesses.
• The patient is participating in another clinical study involving an investigational treatment or participated in one in the previous 4 weeks.
• Patients whose participation in the study may be influenced (employment relationship with the research center or sponsor, inmates, etc.).